The present invention is directed to a system for aerosolizing powders for use in powder spray painting and other powder spray applications. In particular, the present invention is directed to a system for generating an air stream of aerosolized fine powder by forming a turbulent powder cloud from which powder is extracted, which results in a powder spray to a workpiece having a constant powder mass deposition rate.
A steady flow rate of unagglomerated powder particles from the powder spray device is essential to the formation of a smooth, uniform-thickness powder layer on the substrate to be painted in powder spray paint applications. Conventionally, fluidized beds or vibrating troughs have been used to feed powder into the powder spray system. For example, U.S. Pat. No. 5,745,954 to Shutic discloses a powder painting system including a fluidized powder bed. In the disclosed device, powder is fed into a powder feed hopper and falls to the fluidized bed at the bottom of the hopper. The fluidized bed is maintained through the use of pressurized air inlets in the floor of the hopper and rotating baffles. Suction tubes at the top of the hopper extract powder from the fluidized bed.
U.S. Pat. No. 5,654,042 to Watanabe et al. also discloses a powder coating system utilizing a fluidized bed. To improve performance of the device, Watanabe et al. also discloses the use of low-pressure gas pulses directed counter to the normal flow direction of the powder out of the hopper. These low-pressure gas pulses create microvibrations within the powder intake to alleviate adherence and cohesion among powder particles at the pump inlet.
Other prior art devices have attempted to solve the problem of uniform powder flow without fluidized beds. U.S. Pat. No. 5,752,788 to Crum discloses a powder spray device using a vaned impeller to distribute powder before it is delivered to a powder spray gun. The device includes a control system to adjust the rate at which powder is metered to the powder spray gun so that the mass of powder exiting to the powder spray gun remains relatively constant.
In addition, U.S. Pat. No. 4,116,367 to Kataoka et al. discloses an apparatus for supplying powder to a continuous metal casting mold. In this device, powder is fed down into a hopper with compressed air openings along its sides. The powder falls from the hopper through a hole in the bottom, and enters an intake section where the powder is pushed upward by air nozzles. The powder is then sent by a screw to a horizontal air nozzle, which pushes the powder toward the spray nozzle device.
None of these devices is completely satisfactory in operation for steady delivery of powders, and this problem remains as one of the primary difficulties in powder spray painting. In particular, these devices are unable to create a flow of powder in which powder is for the most part separated into individual particles prior to delivery of the powder to the powder sprayer.